Lighting
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LIGHTING JEFF CHASTINE 1 WHAT IS LIGHT? • A very complex process • Find a dark area – how is it being lit? • Light bounces (mirrors, shiny objects) • Light refracts through other media (water, heat) • Light comes from everywhere (Global Illumination) • Light bounces off of lakes in weird ways (Fresnel effect) • http://en.wikipedia.org/wiki/File:Global_illumination.JPG THUS • We’re forced to make approximations • Tradeoff between time and realism • “If it looks good, it is good” – Michael Abrash JEFF CHASTINE http://darrentakenaga.com/3d.html TWO COMPONENTS • Light Source Properties • Color (Wavelength(s) of light) • Shape • Direction • Object Properties • Material • Geometry • Absorption GLOBAL EFFECTS shadow multiple reflection translucent surface 4 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? P0 Directional light - position doesn’t matter - triangle is almost fully lit š P1 P2 JEFF CHASTINE 5 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? P0 š (Triangle less lit) P1 P2 JEFF CHASTINE 6 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? P0 š P1 P2 JEFF CHASTINE (Little to no light hits the surface) 7 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? P0 š (Directional light) P1 P2 JEFF CHASTINE 8 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? P0 š šæ (Directional light) P1 P2 JEFF CHASTINE 9 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? Lesson learned: Lighting depends on angles between vectors! P0 š šæ (Directional light) P1 P2 JEFF CHASTINE 10 A BASIC LIGHTING CONCEPT • How can we determine how much light should be cast onto a triangle from a directional light? intensity = acos(š ā šæ) P0 š šæ (Directional light) P1 P2 JEFF CHASTINE Assuming N and L are normalized, and NāL isn’t negative 11 BASIC LIGHTING • Four independent components: • Diffuse – the way light “falls off” of an object • Specular – the “shininess” of the object • Ambient – a minimum amount of light used to simulate “global illumination” • Emit – a “glowing” effect Only diffuse JEFF CHASTINE 12 BASIC LIGHTING • Four independent components: • Diffuse – the way light “falls off” of an object • Specular – the “shininess” of the object • Ambient – a minimum amount of light used to simulate “global illumination” • Emit – a “glowing” effect Diffuse+Specular JEFF CHASTINE 13 BASIC LIGHTING • Four independent components: • Diffuse – the way light “falls off” of an object • Specular – the “shininess” of the object • Ambient – a minimum amount of light used to simulate “global illumination” • Emit – a “glowing” effect Diffuse+Specular+Ambient Ambient JEFF CHASTINE 14 BASIC LIGHTING • Four independent components: • Diffuse – the way light “falls off” of an object • Specular – the “shininess” of the object • Ambient – a minimum amount of light used to simulate “global illumination” • Emit – a “glowing” effect D+S+A+Emit JEFF CHASTINE Note: emit does not produce light! 15 INTERACTION BETWEEN MATERIAL AND LIGHTS • Final color of an object is comprised of many things: • The base object color (called a “material”) • The light color • Example: a purple light on a white surface • Any textures we apply (later) • Materials and lights have four individual components • Diffuse color (cd and ld) • Specular color (cs and ls) • Ambient color (ca and la) • Emit color (ce and le) • cd * ld = [cd.r*ld.r , cd.g*ld.g , cd.b*ld.b] // R, G, B JEFF CHASTINE 16 LIGHT SOURCE DIRECTION • In computer graphics, we usually treat lights as rays emanating from a source. The direction of these rays can either be: • Omni-directional (point light source) • Directional angle (spotlights) • Directional (parallel rays) GENERAL LIGHTING • Primary vectors • l – the incoming light vector • n – the normal of the plane/vertex • r – the reflection vector • v – the viewpoint (camera) v n l r θ JEFF CHASTINE θ 18 DIFFUSE TERM • Contribution that a light has on the surface, regardless of viewing direction. • A ray of light coming in has an equal chance of being reflected in any direction. • What are some ideal diffuse surfaces? LAMBERTIAN REFLECTANCE (DIFFUSE COMPONENT) • Light falling on an object is the same regardless of the observer’s viewpoint • Good for rough surfaces without specular highlights • ššššš_šššššššššš¢š š = š ā š ∗ šš ∗ šš where š and š are normalized n l θ JEFF CHASTINE 20 LAMBERTIAN REFLECTANCE (DIFFUSE COMPONENT) • Light falling on an object is the same regardless of the observer’s viewpoint • Good for rough surfaces without specular highlights • ššššš_šššššššššš¢š š = š ā š ∗ šš ∗ šš where š and š are normalized 3 parts (R, G, B) scalar n l θ Note: final_colordiffuse has R, G, B JEFF CHASTINE 21 LAMBERTIAN REFLECTANCE (DIFFUSE COMPONENT) • Technically, it should be: ššššš_šššššššššš¢š š = max(š ā š, 0) ∗ šš ∗ šš n l θ JEFF CHASTINE 22 BLINN-PHONG REFLECTION (SPECULAR COMPONENT) • Describes the specular highlight and is dependent on viewpoint v • Also describes a “half-vector” h that is halfway between v and l h v n r l θ JEFF CHASTINE θ 23 BLINN-PHONG REFLECTION (SPECULAR COMPONENT) • ā = š£ + š - which is really Blinn’s contribution to the original Phong model h v n r l θ θ Note: vectors should be normalized JEFF CHASTINE 24 BLINN-PHONG REFLECTION (SPECULAR COMPONENT) • Our final specular equation is: ššššš_šššššš = (š ā ā) š ∗ šš ∗ šš h v n r l θ JEFF CHASTINE θ 25 DETERMINING š ššššš_šššššš = (š ā ā)š ∗ šš ∗ šš • Realize that š ā ā will always be < 1.0, so raising it to a power will make it smaller • š is the “shininess” factor • It relates to the size of the specular highlight s = ~1 JEFF CHASTINE s = ~30 s = ~255 26 AMBIENT AND EMIT COMPONENTS • Ambient: • Used to simulate light bouncing around the environment (global illumination) • Real world is far too complex for real time, so just add a little light! • Emit: • Used to make the object “glow” • Does not emit light!!! • Both: • Independent of viewpoint • Super easy to calculate ššššš_šššššššššššš” = šš + šš ššššš_ššššššššš” = šš + šš JEFF CHASTINE 27 FINAL COLOR • To determine the final color (excluding textures) we sum up all components: + final_colordiffuse final_colorspecular final_colorambient final_coloremit final_color http://en.wikipedia.org/wiki/Phong_reflection_model JEFF CHASTINE 28 WHAT ABOUT MULTIPLE LIGHTS? • Calculate final colors and sum them all together • Assuming results are in f [ ] and there are count number of lights ššš¢šš” ššššš_ššššš = (š š š , š š š , š š š , š[š]š ) š=1 JEFF CHASTINE 29 COMMON KINDS OF LIGHTS • Point light • Directional Light • Spot Light • Area Light • Interesting fact: • Lights cannot be seen! • Only their effects • We can light per vertex (fast) or per fragment (slower) JEFF CHASTINE 30 POINT LIGHTS • These lights have a position in 3D space • Sometimes called a “Lamp” • Light emanates from the light in all directions • Distance d determines brightness (“attenuation”): ššš”ššš šš”š¦ = 1/š 2 JEFF CHASTINE Here, per fragment lighting used 31 POINT LIGHTS • These lights have a position in 3D space • Sometimes called a “Lamp” • Light emanates from the light in all directions • Distance d determines brightness (“attenuation”): ššš”ššš šš”š¦ = 1/š 2 JEFF CHASTINE Here, per vertex lighting used 32 DIRECTIONAL LIGHTS • Are infinitely far away • position in NO WAY matters • Have only direction • All objects are lit evenly • Sometimes called a “Sun” JEFF CHASTINE 33 SPOTLIGHTS • Point light source • Conical in shape JEFF CHASTINE 34 SPOTLIGHTS • Point light source • Conical in shape • Have: • An inner and outer cone angle • Umbra – areas that are fully in shadow • Penumbra – areas that are in partial shadow • Note: There’s an ambient light JEFF CHASTINE 35 AREA LIGHTS • A “surface” lights objects • Has a position and direction • Provides for a smoother drop off than point • Larger surface == smoother shadows • Expensive to calculate JEFF CHASTINE 36 THE END! What you’ll see if you don’t glEnable(GL_LIGHTING)
